1. The Field of the Invention
This invention relates in general to transducers, and in particular to directional boundary-type microphones. In this particular field of directional boundary-type microphones, the emphasis is upon extending the art, to create a device combining the pick-up distance of the fixed-boundary type with an adjustable pick-up pattern and having ease of carriage and storage.
2. Background Information
In live theatre, a problem exists in being able to clearly hear and understand the actors' voices (used in this document, the word actor also applies to actresses, as well as musicians and other desired sound sources). In addition to all the other items needed for good sound reinforcement in a theatre, it is preferred to have microphones that can provide the following qualities:
Easily adjustable horizontal angle of pick-up with little change in level, to allow for the variations in microphone placement, scenery and unwanted noise sources. Horizontal adjustment extends from narrow to very wide, so that actors are not limited in their movement about the stage.
Portability for ease of storage and shipping.
Narrow vertical angle of pick-up, rejecting sound from any loudspeakers located over the stage, reducing possible feedback. With some means of adjustment allowing for pickup of actors physically close to the microphone.
High intelligibility. The microphone should pick up direct sounds of the actors' voices, and reject voice reflections, reducing comb-filtering. Comb-filter refers to the appearance of a frequency response graph, produced when signals of the same frequency but differing path lengths combine.
Not just high directivity, rather, higher sensitivity within acceptance pattern; very low sensitivity in rejection pattern. Sound pick-up from wanted directions should be consistent and smooth with very little change in level versus angle. Transition from the accepting side to the rejecting side should be abrupt with very little pick-up over the entire rejecting side. This implies use of boundary pattern microphones which have a polar response essentially even from boundary to boundary, with a sharp reduction in response behind the boundary. This is in contrast to limacon pattern (cardioid, figure-of-8, hyper-cardioid) microphones, which have an acceptance pattern that gradually changes to a very small rejection pattern. (A limacon is a unicursal curve of the fourth order, investigated and named by the mathematician Blaise Pascal.) This gradual change means limacon pattern microphones do not perform as well as boundary-type microphones. The ratio of on-axis energy response to random sounds of an ideal limacon pattern microphone is, at best, 6 dB; while that of an ideal triple-boundary microphone covering a 90-degree horizontal and 60-degree vertical pattern exceeds 18 dB. The combination boundary, hyper-cardioid (CROWN PCC) is, at best, 12 dB. Limacon pattern microphones don't cover all of their rated angle of pick-up at full sensitivity as do boundary microphones.
A high signal-to-noise ratio, necessary because actors may be at a distance from the microphone. High directivity increases the signal-to-noise ratio by giving more desired signal to the transducer.
Long-distance pick-up. The sound reinforcement system should have good gain-before-feedback, meaning high amplification before oscillation occurs. This is necessary in order to pick-up an actor's voice at a distance. One of the sound system's dependent variables is a microphone having good rejection of sound coming from the loudspeaker as well as surfaces reflecting sound from the loudspeaker, while having good acceptance of sound coming from the actor.
Smooth frequency response. The amplitude response relative to frequency should exhibit low variation over its rated frequency spectrum. To accomplish this it is necessary to reduce comb-filtering.
3. Description of Related Art
Wireless microphones are used in theatres at considerable additional expense and with less reliability due to their dependence on batteries and need for clear-channel radio frequencies, though they do provide high gain before feedback.
Another microphone used in theatres is the "shotgun" type, with a very narrow pick-up pattern (like a spotlight), requiring many units and/or an operator.
4. Prior Art
One of the best practical devices of Prior Art is the "PCC floor-mounted cardioid boundary microphone by CROWN INTERNATIONAL", which has half the pick-up distance of the invention which is the subject of this application.
Other boundary microphones. See SOUND SYSTEM ENGINEERING (SSE), by Don and Carolyn Davis, ISBN O-672-21857-7 pages 296 through 303 and 625 through 629. This publication shows many designs of multi-boundary microphones used with varying degrees of success (though not practical to manufacture) for live theatre and music, including:
Fixed-panel, floor-mounted, multi-boundary microphones (shown on page 626, FIG. VIII-4 of SSE) perform as well as this invention, but are not adjustable, nor easily carried, shipped or stored due to their bulk and/or delicate construction.
The adjustable-angle, boundary microphones (shown on pages 302, 303 and page 627 FIG. VIII-6 of SsE) in comparison to the said fixed-panel types, have less directivity due to having only two boundaries per transducer. One angle of coverage is not adjustable, and at 180 degrees is too large, giving less directivity. If the floor is used as a third boundary, the lack of a seal to the floor compromises the directivity enhancement and the location of the transducer incurs comb-filtering penalties. The angle is adjustable in only one plane, with no means to fix the adjustment.
Whatever the precise merits, features and advantages of the above cited references, none of them achieves or fulfills all of the below-stated objectives. It is clear that to advance the art, it is necessary that a combination of the fixed-panel and adjustable-panel boundary microphones be implemented. This was not possible with existing designs due, to lack of a method to both hinge the panels for adjustability and collapsibility, and simultaneously seal the third side (floor) to the two hinged sides.